High intensity attosecond beamline for XUV pump XUV probe measurements with photon energies up to 150 eV

This paper presents a newly constructed high-intensity attosecond beamline capable of generating stable, isolated extreme ultraviolet pulses with energies up to 150 eV and 55 nJ pulse energy, specifically designed to enable nonlinear XUV pump-probe spectroscopy with sub-micrometer spatial resolution.

The Gist

Imagine trying to take a photograph of a hummingbird's wings in mid-flight. If you use a standard camera, the wings will just look like a blur. To freeze that motion, you need a flash so fast it's over before the wing even moves a fraction of a millimeter.

Now, imagine that hummingbird is an electron inside an atom, and it's moving so fast that even a "fast" camera flash is too slow. To see electrons dance, we need a flash that lasts for attoseconds. An attosecond is to a second what a second is to the age of the universe. It is an almost incomprehensibly tiny slice of time.

This paper describes the construction of a brand-new, super-powerful "camera flash" machine designed specifically to freeze these electron motions. Here is the story of how they built it, explained simply.

1. The Problem: The "Dim Flash"

For years, scientists could make these attosecond flashes using a process called High-Harmonic Generation (HHG). Think of this like hitting a drum to make a sound. You hit the drum (shoot a laser at a gas), and it vibrates, creating a higher-pitched sound (an X-ray flash).

However, there was a major problem: The flashes were too weak.

• Most existing machines were like a tiny LED flashlight. They could take a picture, but the light was so dim that you couldn't do complex experiments.
• To study how atoms react when two flashes hit them at once (a "pump-probe" experiment), you need a light so bright it can actually knock electrons out of atoms or change their behavior. The old "LEDs" just weren't strong enough.

2. The Solution: The "Stadium Light"

The team at Umeå University and Lund University built a new machine that turns that tiny LED into a stadium floodlight.

They used a massive laser system called LWS100. Imagine a laser so powerful it could cut through steel, but they had to be very careful with it.

• The Trick: They didn't use the whole laser beam. They used a "cookie cutter" (an iris) to take a specific slice of the beam.
• The Gas: They shot this slice of laser into a cloud of Neon gas.
• The Result: The laser hit the neon atoms so hard that the atoms screamed back in the form of a super-bright, ultra-short flash of light (Extreme Ultraviolet or XUV).

Because they used such a powerful laser and a very long, precise setup (like a 22-meter long tunnel), they managed to create flashes with 55 nanojoules of energy.

• Analogy: If previous machines were like a single firefly, this new machine is like a thousand fireflies flashing in perfect unison. It's roughly 100 times brighter than the standard machines used in labs today.

3. The "Split-and-Delay" Stage: The Magic Mirror

To study how electrons move, scientists need to hit the atom with one flash (the "pump") to start a reaction, and then hit it again with a second flash (the "probe") a tiny fraction of a second later to see what happened.

The team built a special Split-and-Delay Stage.

• The Analogy: Imagine a river of light flowing toward a fork. They used a special mirror with a tiny gap in the middle to split the river into two streams.
• The Precision: One stream goes straight; the other hits a mirror that can move back and forth. By moving that mirror by the width of a human hair, they can delay the second flash by a few attoseconds.
• The Stability: This mirror is so stable that if you watched it for an hour, it would wobble less than the width of a single atom. This ensures the timing is perfect.

4. The "Super-Resolution" Trick

Sometimes, the laser pulse is a little too long, like a slightly blurry photo. The team used a trick called Temporal Super-Resolution.

• The Analogy: Imagine a long, wavy rope. If you cut out the middle section of the rope, the remaining ends snap together to form a shorter, tighter knot.
• By filtering out the "middle" colors of the laser light, they made the laser pulse shorter and sharper. This made the resulting X-ray flash even more isolated and powerful, perfect for capturing the fastest electron movements.

5. The "Ion Microscope": Seeing the Aftermath

Once the bright flash hits the target (like Xenon gas), it knocks electrons off the atoms, turning them into ions (charged particles).

• The team built an Ion Microscope. Think of this as a high-speed camera that doesn't take pictures of light, but of electrically charged particles.
• When the flash hits, the ions fly out. The microscope catches them and projects a magnified image of where they went. This tells the scientists exactly what the atom looked like after the flash hit it.

Why Does This Matter?

This new beamline is a game-changer for physics.

• Before: Scientists could mostly watch electrons move, but they had to use a strong laser to "hold" the atom in place, which sometimes changed the experiment.
• Now: With these super-bright, isolated flashes, scientists can hit atoms with only X-ray light. They can watch electrons move freely without the "interference" of a strong laser holding them down.

In summary: This paper describes the construction of the world's most powerful "strobe light" for the atomic world. It allows scientists to finally freeze-frame the fastest events in nature, opening the door to understanding how electrons behave in solar cells, new computer chips, and chemical reactions, potentially leading to revolutionary new technologies.

Technical Summary

1. Problem Statement

The field of attosecond physics has advanced significantly, yet experimental facilities capable of performing XUV pump–XUV probe spectroscopy remain scarce. Most existing High-Harmonic Generation (HHG) sources suffer from low conversion efficiency, resulting in pulse energies in the sub-fJ to pJ range. These low energies are insufficient to drive nonlinear interactions in the XUV regime without the presence of a strong fundamental laser field, which can alter or damage the sample. While Free-Electron Lasers (FELs) offer high intensity, they are expensive, rare, and typically lack the broad bandwidth required for isolated attosecond pulses in the soft X-ray region. There is a critical need for a laboratory-scale source that delivers high-energy, isolated attosecond pulses (tens of nJ) with photon energies up to 150 eV to enable pure XUV pump–XUV probe experiments.

2. Methodology

The authors constructed a dedicated beamline at the Relativistic Attosecond Physics Laboratory (REAL) at Umeå University, utilizing a novel driving laser and optimized HHG geometry.

• Driving Laser System: The source uses the Light Wave Synthesizer 100 (LWS100), an Optical Parametric Synthesis (OPS) system capable of generating <4.5 fs pulses with up to 480 mJ energy. For HHG, the energy is clipped to ~120 mJ (10 Hz repetition rate) to optimize the focal spot size while maintaining high peak power.
• Temporal Characterization: A chirp-scan setup using a BBO crystal and an acousto-optic programmable dispersive filter (DAZZLER) compresses the laser pulses to the Fourier limit (~4.3 fs, ~1.6 optical cycles) to facilitate the generation of isolated attosecond pulses without additional gating techniques.
• HHG Generation:
  • Medium: Neon gas is used in both jet and gas cell configurations.
  • Geometry: A 22 m focal length is employed to create a large focal spot (425 µm FWHM), ensuring phase matching is maintained at high intensities.
  • Optimization: Extensive numerical simulations (solving propagation equations with time-dependent Schrödinger equation data) were used to optimize gas pressure and cell length. The experimental optimum was found to be a 14 cm gas cell at ~2.6 mbar.
• Beamline Components:
  • Split-and-Delay Unit (SDU): A grazing-incidence gold split mirror system with piezo actuators allows for precise temporal delay control (range ~20 fs, jitter <25 as) between pump and probe pulses.
  • Filtering: Motorized filter wheels with thin-film metallic filters (Zr, Pd, Al, Si) select specific spectral windows (e.g., Zr window: 65–150 eV).
  • Focusing: An ellipsoidal gold-coated mirror (8° grazing incidence, 125 mm focal length) focuses the XUV beam to a <6 µm spot. A spherical back-reflecting multilayer mirror is also available for smaller spots (~1 µm) but with lower throughput.
  • Detection: The beamline features an Ion Microscope (for spatially resolved ion imaging and Time-of-Flight analysis) and a Velocity Map Imaging (VMI) spectrometer (for electron momentum measurements up to 240 eV).
• Temporal Super-Resolution: To further isolate pulses and broaden the spectral continuum, the authors applied spectral amplitude modulation (cutting the laser spectrum around the center), reducing the driver pulse duration to 4.0 fs.

3. Key Contributions

• High-Energy Source: Demonstration of a laboratory-scale source delivering 55 nJ of pulse energy in the Zr window (65–150 eV), with typical on-target energies >10 nJ. This is >100 times higher than standard kHz HHG systems.
• Isolated Attosecond Pulses: Generation of well-isolated attosecond pulses at the highest photon energies (up to 150 eV) without complex gating, achieved via sub-two-cycle driving pulses.
• Integrated Beamline: A complete facility featuring a split-and-delay stage for pump-probe experiments, multiple focusing options, and advanced detectors (Ion Microscope and VMI) for characterizing nonlinear ionization.
• Validation: Comprehensive agreement between numerical simulations and experimental results regarding conversion efficiency, beam divergence, and spectral properties.

4. Results

• Pulse Energy & Stability: The source delivers up to 55 nJ in the 65–150 eV range with a stability of 5–10% (RMS).
• Beam Quality: The XUV beam is highly collimated with a divergence of ~70 µrad. The focused spot size is approximately 5.7 µm (FWHM) using the ellipsoidal mirror, achieving peak intensities of ~10¹⁴ W/cm².
• Spectral Performance: The spectrum extends to a cutoff of ~150 eV. Under optimal conditions, the harmonic lines in the cutoff region vanish, forming a continuum that indicates the generation of isolated attosecond pulses.
• Temporal Calibration: The split-and-delay stage was calibrated using a He-Ne laser, achieving a calibration factor of 274.5 as/µm and an out-of-loop RMS temporal jitter of 20–25 as.
• Nonlinear Interaction: The intensity is sufficient to drive nonlinear ionization in Xenon (e.g., Xe → Xe⁴⁺), which was verified using the ion microscope. The ellipsoidal mirror provides ~27 times higher energy throughput compared to spherical multilayer mirrors for the Zr window.
• Super-Resolution: Applying temporal super-resolution broadened the XUV continuum by 35%, though it resulted in a ~10% reduction in total XUV energy due to the trade-off between pulse duration and intensity.

5. Significance

This work represents a major step forward in attosecond science by providing a robust, high-intensity platform for XUV pump–XUV probe experiments.

• Elimination of Laser Field Artifacts: By using intense XUV pulses for both pump and probe, researchers can study electron dynamics without the perturbing influence of a strong fundamental laser field, which is crucial for observing intrinsic atomic and molecular processes.
• Access to Nonlinear Regime: The high pulse energies enable the study of nonlinear XUV phenomena (e.g., multi-photon ionization, high-harmonic generation in the XUV) which were previously inaccessible in laboratory settings.
• Broad Applicability: The beamline's versatility (tunable photon energy, isolated pulses, high spatial resolution) makes it an ideal testbed for investigating ultrafast electron dynamics in atoms, molecules, and solids, paving the way for new discoveries in quantum dynamics and material science.